Project Summary/Abstract Cardiac alternans is characterized by a beat-to-beat alternation in membrane potential that is known to trigger cardiac reentry in experiments and has been correlated with risk for clinical tachyarrhythmias. In recent years progress has been made in illuminating the mechanisms of alternans. However, significant uncertainty remains. Studies have suggested that alternans may result from dynamical instabilities in either or both membrane voltage or calcium cycling. More specifically, the two proposed mechanisms are: (i) sarcolemmal ion current dynamics cause repolarization alternans, which in turn produces calcium alternans, and (ii) sarcoplasmic reticulum calcium uptake and release mismatch causes calcium alternans, which in turn produces repolarization alternans via calcium coupling to sarcolemmal currents. For many years, mechanism (i) was thought to explain the occurrence of alternans. Specifically, the action potential duration (APD) restitution function was used to predict alternans based on the hypothesis that restitution slopes >1 dictate that intrinsic APD variations will be amplified into alternans. However, evidence has mounted that this relationship often does not hold. In contrast, evidence for mechanism (ii) has accumulated, pushing that theory to the forefront. All that being said, in spite of widespread interest in identifying the mechanism of alternans, we hypothesize that there is not necessarily one generic mechanism for alternans, but rather that mechanisms (i) and (ii) play varying, but quantifiable, roles for different cardiac cell types. To investigate this hypothesis, we will use synergistic computational and experimental approaches: 1. To quantify the sensitivity of cellular alternans to action-potential morphology. 2. To quantify the cell-type dependence of whole-cell and subcellular alternans mechanisms in vitro. 3. To investigate the tissue-level implications of cellular alternans mechanisms using computational modeling and ex vivo optical mapping experiments. By spanning several spatial scales, from subcellular to tissue-level, the synergistic computational and experimental studies proposed here will help to provide an integrated understanding of alternans dynamics. Furthermore, identification of the mechanisms of alternans will help advance our understanding of alternans arrhythmogenesis, which may have clinical implications.